Take the Pressure Down: Understand Equilibrium!

Understanding the concepts behind equilibrium can really pay off in the GAMSAT. This is because if you are asked a straight out equilibrium question, such as “what is the equilibrium expression for the following equation…” it can be relatively easy marks.

But also because understanding equilibrium is a must if you want to nail some more tricky topics that involve equilibrium concepts, such as acids and bases and solubility product questions.

One of the key understandings when it comes to the concepts that underpin the basics of equilibrium, is knowing how the following will effect the position of the equilibrium and the actual value of the equilibrium constant:

  • concentration changes
  • pressure
  • temperature
  • addition of a catalyst

The following videos from the Virtual School are really great visuals to help you really nail those basic concepts of equilibrium. They both use Le Chatelier’s principle to explain the effects of the above factors on a system at equilibrium.

The first video explains how changes in concentration and pressure affect equilibrium:

The second video explains how changes in temperature or addition of a catalyst affect equilibrium:

In summary:

  • Concentration and pressure changes will shift the position of the equilibrium, and Le Chatelier’s principle says that the equilibrium will move to oppose that change, in order to re-establish equilibrium and maintain the same value of the equilibrium constant, K.
  • An increase or decrease in temperature will favour one side of the reaction over the other. The actual value of the equilibrium K will change.
  • Addition of a catalyst does not affect the position of the equilibrium OR the value of the equilibrium constant, K. It merely increases the rate of both the forward and back reactions, hence equilibrium is reached faster.

To determine the direction that the equilibrium will shift, you’ll need to know the following about the chemical reaction:

  • Concentration changes: identify if the reagent is experiencing an increase or decrease in concentration and if it is a reactant or product
  • Pressure changes: identify the number of moles of gas on each side of the equation
  • Temperature: identify the energetics of the reaction, is it an exothermic or endothermic reaction?

For a more indepth explanation, you can always check out this comprehensive explanation on chemguide.co.uk.

If you’re still feeling overwhelmed because the GAMSAT is just getting closer and closer…perhaps it’s time to breathe and sing a little song to yourself:

“Take the pressure down
Cause I can feel it, It’s rising like a storm
Take hold of the wheels and turn them around
Take the pressure down…”

Good luck 😉

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